E-cigarette use damages healthy cells and increases the risk of respiratory infections, a new study finds, adding to growing concerns about the negative health effects of ‘vaping’.

The use of electronic cigarettes, or e-cigarettes, is on the rise nationwide, with more than 40 million Americans having already tried the product at least once. Marketed as a safer alternative to traditional tobacco products, e-cigarettes are particularly appealing to current and former smokers, while use among teenagers has also increased sharply in the past few years.

But as the product’s popularity rises, so do the concerns about its potential impact on public health. Research shows, for instance, that secondhand smoke from e-cigarettes actually contains higher levels of toxic metals than conventional cigarette smoke.

In this latest study, researchers from National Jewish Health in Denver tested the liquid used in e-cigarettes, sometimes called e-liquid. To examine the health effects, the team put cells from the airways of healthy, young, non-smokers in one end of a device and an e-cigarette in the other. They found that the liquid quickly damaged healthy cells, resulting in lasting effects.

“It increased the level of viral infection inside the cells,” said Dr. Hong Wei Chu, who led the study. In fact, they found that after just ten minutes of exposure, the cells were already damaged. What’s more, the observed damage lasted 24 hours or longer. The study showed it didn’t matter if the liquid contained nicotine or not, the liquid itself did the damage.

The researchers say the findings are especially troubling since some e-cigarettes are flavored to appeal to younger users. “When you flavor them that way, not only are they appealing, but, falsely, the user sees them as ‘Oh, no big deal. They’re not bad for me,'” said Dr. Chu.

Over the next week, millions of Americans will make New Year’s Resolutions to quit smoking. Many will turn to e-cigarettes as a method to help. Over the last year, as sales topped $3.5 million, concerns about safety have prompted leading health authorities to issue warnings about the product. In July, the World Health Organization found there was not enough evidence to determine if electronic cigarettes really help people quit smoking; indeed, some studies even suggest that e-cigarettes may encourage conventional cigarette use in adolescents. The World Lung Foundation and the American Heart Association, among others, have recommended tighter regulation of e-cigarettes due to those growing concerns, calling for a ban on sales to minors and further research to study the health effects.